The present invention relates to a film for high heat-sensitive stencil paper and, particularly, to a film for high heat-sensitive stencil paper which has an excellent perforation sensitivity and an excellent printing quality, i.e., resolution and density of a printed image.
As a heat-sensitive stencil paper, a laminate composed of a film of a thermoplastic resin such as polyester and thin porous paper has been known. The film used for this purpose is required to have the following properties.
(1) The film is required to have a high thermal perforation sensitivity. The film is required to melt with a small quantity of heat and to have an adequate heat shrinkage which enables openings of an appropriate size for producing a clear printed image to be formed. PA0 (2) The film is required to have high strength and elasticity enough to withstand operations of laminating with thin porous paper and printing. In addition, even if the operation includes a heating step, the film is required not to cause a problem such as curling of the stencil paper due to heating. PA0 (3) The film is required to have good gradation in thermal perforation. The gradation of a printed image is deteriorated if a portion other than the portions to be perforated is melted due to the influence of the neighboring perforation. The film is required to have good thermal perforation property which can discriminate between the portions to be perforated and the portions not to be perforated.
In addition to these requirements, the film is required to be excellent in the handling property at the time of the production of the film and the formation of the stencil paper. For example, the stretching property is so good as not to cause a trouble such as breakage during production of the film, and that the wind-up property and slittabiltiy are so good as not to produce a crease on the film or as not to cause weaving. It is also necessary that no trouble is caused at the film winding step or the film running step during the formation of stencil paper.
As a film (biaxially oriented film formed from a thermoplastic resin) for heat-sensitive stencil paper, there have been proposed a film improved in printing characteristics by defining the thermal properties (Japanese Patent Application Laid-Open (KOKAI) No. 62-149496 (1987)), a film defined by the surface roughness and the number of protuberances on the film surface (Japanese Patent Application Laid-Open (KOKAI) No. 63-227634 (1988)) and films defined by the heat shrinkage properties (Japanese Patent Application Laid-Open (KOKAI) Nos. 62-282983 (1987), 63-160895 (1988), 63-312192 (1988), and 3-30996 (1991)) .
However, these proposed films cannot be said to be satisfactory for heat-sensitive stencil paper, especially, in the points of perforation sensitivity and printing quality.
As a result of various studies undertaken by the present inventors so as to solve the above-described problems, it has been found that a biaxially oriented film formed from a polyester having a specific composition and having the heat shrinkage and the heat shrinkage stress which satisfy specific conditions, is suited as a base film for heat-sensitive stencil paper. On the basis of this finding, the present invention has been achieved.